The invention relates to a diesel fuel containing an additive which improves the combustion of soot.
In addition to the pollutants which are also formed in spark-ignition engines, diesel engines emit soot particles which, for some years, have been viewed very critically. Studies by animal experiments have shown that diesel exhaust gas has a carcinogenic potential. In 1987, diesel exhaust gas was therefore included as a carcinogenic working material in the list of maximum allowable concentrations.
To reduce the particle emission in the exhaust gases from diesel engines, it is now part of the state of the art to precipitate the particles formed during the combustion process in a downstream filter device and to oxidize them therein. Predominantly used nowadays as such filter devices are monolithic ceramic bodies of honeycomb structure or, for example, wound ceramic filters in which a yarn of ceramic fibers has been applied to perforated steel tubes. Fairly good precipitation of the soot particles is achievable by means of such filter bodies. What has not yet been satisfactorily solved is the absolutely necessary regeneration of the particle filters. Without additional measures, the soot precipitated in the particle filter is oxidized at a sufficiently fast rate only at temperatures above 600.degree. C. In normal running of the motor vehicle, however, such high exhaust gas temperatures are only very rarely reached. With increasing filter loading, the exhaust gas back-pressure rises steeply and impairs the combustion behavior and the power of the engine to considerable extent. Above all, however, there is a risk of a filter too heavily coated with soot particles being overstressed during a regeneration by the heat released during the exothermic oxidation of soot and hence being damaged.
Various measures have already been disclosed in the state of the art, which are intended to allow a regeneration of the particle filter even at lower temperatures. For this purpose, it has been suggested to coat the ceramic support material of the particle filter with a catalytically active substance (German Patent Document DOS 3,232,729). However, the coatings hitherto used have proved to be not sufficiently effective. In addition, there are reservations on toxicological grounds against certain suggested coating substances, for example the vanadium oxide according to the specification quoted above. It is also already known to arrange an additional burner next to the particle filter, which is intended to burn the particle filter free of the precipitated soot under control. Direct heating of the particle filter is also already part of the state of the art (German Patent Document DOS 3,538,155). Likewise, it has already been described to add a catalytically active substance in a controlled amount to the exhaust gas stream for the combustion of the soot (German Patent Document DOS 3,325,391). To reduce the soot content in the diesel engine exhaust gases, organic boron compounds (German Patent Document DOS 2,340,522), which were admixed to the diesel fuel, or copper salts and ammonium salts (German Patent Document DOS 3,325,391) or perchlorates (German Patent Document DOS 3,436,351) have also already been recommended as additives, which are metered into the exhaust gas upstream of the soot filter. The results achieved thereby have, however, not been convincing and, with some of the suggested compounds, an additional pollutant emission injurious to health into the environment cannot be excluded. Thus, it is also shown by "Automobiltechnische Zeitschrift" 86 (1984) 2, page 76, left-hand column, that fuel additives for preventing an emission of soot have been developed which consist of metal-organic compounds of the alkaline earth metals or of alkaline earth metal sulphonates. It is explicitly pointed out here, however, that metal oxides were then formed in the combustion, which caused increased engine wear, and an increase in the toxicity of the exhaust gases could also not be excluded.
It is therefore an object of the invention to provide a diesel fuel containing an additive which improves the combustion of soot which is deposited on a downstream particle filter, in order to reduce the pollutant emission in the combustion exhaust gases from diesel engines by burning off the soot which has been precipitated on the particle filter, it being intended to reduce the ignition temperature of the soot, so that regeneration of the particle filter at low temperatures is possible. At the same time, the disadvantages indicated above should be avoided and, moreover, no additional pollutant emissions damaging to the environment should arise in the diesel engine exhaust gases.
According to the invention, the stated object is achieved by means of lithium, sodium, or potassium salts of organic compounds as an additive. In especially preferred embodiments, the metal salts at the following organic compounds is added, singly or as a mixture:
(a) of an aliphatic alcohol of the general formula CH.sub.3 --X--OH, X being an alkyl group having 1 to 8 carbon atoms, or of a compound isomeric with such an alcohol, or PA1 (b) of an aromatic alcohol of the general formula ##STR1## X being an alkyl group having 1 to 8 carbon atoms, or (c) of a phenol of the general formula ##STR2## X being an alkyl group having 1 to 8 carbon atoms, or (d) of an aliphatic carboxylic acid of the general formula CH.sub.3 --X--COOH, X being an alkyl group having 3 to 16 carbon atoms, or of a compound isomeric with such a carboxylic acid, or PA1 (e) of 1-naphthoic acid, 2-naphthoic acid, phenylacetic acid or cinnamic acid.
In especially preferred embodiments, the metal salt added per liter of diesel fuel contains 0.1 to 50 millimole of alkali metal.
It has been found that an extremely reactive soot is formed in the diesel engine in the combined combustion of the indicated compounds with the diesel fuel. After the soot has been precipitated in the soot filter, the soot particles can be rapidly oxidized even at very low temperatures. In the studies, it has been found that the regeneration temperatures depend very greatly on the speeds and loads at which the engine is run. The engine conditions have a very great influence on the morphology of the soot and hence also the reactivity thereof. Under certain engine conditions, good regenerations of the soot filter are possible even at exhaust gas temperatures below 200.degree. C. In contrast to the additives added to the diesel fuel according to the state of the art, there are no health reservations against the use of the alkali metal salts added according to the invention. No MAC values can be found in the literature, and there are also no indications of a potential carcinogenicity or co-carcinogenicity.
Above all for lithium compounds amongst the alkali metal salts added according to the invention, there are indications to the effect that the course of the combustion is favorably influenced and the emission is already lowered inside the engine. Above all, however, during the combustion process in the presence of the additives studied, a soot is formed which is much more readily oxidizable after precipitation in a particle filter. This has the consequence that very much lower exhaust gas temperatures suffice for the particle filter regeneration and a critical coating with masses of soot in the filter is thus avoided.